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Wednesday, August 10, 2011

Mel, over at Sandy Wildlife has posted a fascinating piece about Dodder Cuscuta europaea, one of the most interesting British native flowering species, that has no root system of its own but parasitises other plants. Her post (highly recommended) is full of fascinating information about the plant and its history but I thought I would add a little about growing it. Some years ago I was given some dodder seed, which I germinated on wet paper towel (germination took about a week), then transferred the spindly seedlings to the soil surface around the base of a stinging nettle Urtica dioica plant in a flower pot. The seedlings elongate quickly and the yellow growing point rotates in a circular motion as it elongates (circumnutation) , until it touches the host plant stem. Once it makes contact there is a delay of a few days and them something remarkable happens...

........ the slender thread swells massively and then coils around the host stem. At this stage it looks more like a reptile than a plant. This transformation takes place because the dodder shoot tip has produced an invasive haustorium that has penetrated the host stem and linked up with its victim's vascular tissue, so now it can divert nutrients from its host to support the new aggressive phase of growth.

After that invasion is very rapid. The dodder branches and wherever it makes contact with the host it 'plugs-in' another haustorium - here you can see haustoria penerating the nettle stem, just a little way up from the bottom of the photo above. It often coils around itself but it seems that the haustoria can't penetrate the plant's own stems.

This is a thin transverse section, just one cell thick, cut through the point of contact between the dodder and nettle stems, seen under the microscope. The dodder stem is the darker tissue, top left, and you can see its haustorium puncturing the outer layers of nettle stem cells. If you look closely (click for a larger image) you can see the tip of the haustorium dividing into finger-like files of cells that are heading towards the host's vascular bundles (labelled V) that are conducting water, minerals and sugars within the nettle.

Once multiple haustoria have established the parasitic dodder grows very rapidly and then....

... flowers prolifically. The small white flowers are produced in clusters. A fascinating plant - a vampire of the vegetable kingdom.